C#

private void button1_Click(object sender, EventArgs e)
{
    CurrentTextBox.Text += "A";
    CurrentTextBox.Capture = true;
}

Control activeFormControl = this.ActiveControl;    // of the Form<br />
Control activePanelControl = panel1.ActiveControl;       // of a Panel

TextBox CurrentTextBox;

private void TextBoxes1To4_Enter(object sender, EventArgs e)
{
    CurrentTextBox = sender as TextBox;

    // remove when testing is complete
    if(CurrentTextBox == null) 
    {
        throw new ArgumentNullException("Illegal use TextBox Enter");
    }

    // example of code that will do something when TextBox is entered
    switch (CurrentTextBox.Name)
    {
        case "textBox1":
            // do something specific to textBox1 getting Focus, being Acive
            // example:
            textBox1.Text = "Entered";
            break;
        case "textBox2":
            break;
        case "textBox3":
            break;
        case "textBox4":
            break;
    }
}

private TextBox _tb = null;

this.textBox1.Leave += new System.EventHandler(this.textBox_Leave);
this.textBox2.Leave += new System.EventHandler(this.textBox_Leave);
this.textBox3.Leave += new System.EventHandler(this.textBox_Leave);

private void textBox_Leave( object sender, EventArgs e ) {
   _tb = sender as TextBox;
}

private void button1_Click( object sender, EventArgs e ) {
   if ( _tb != null ) {
      MessageBox.Show( "The last TextBox was: " + _tb.Name );
   }
}

namespace YOURPROJECTNAMESPACE
{
    public sealed class MyControl : Label
    {
        public MyControl()
        {
            // Allow transparent background
            SetStyle(ControlStyles.SupportsTransparentBackColor, true);
            // Default to transparent background
            BackColor = Color.Transparent;
        }

        public Image ControlImage { get; set; }

        protected override void OnPaint(PaintEventArgs e)
        {
            // Draw image, sized to control
            if (ControlImage != null)
            {
                e.Graphics.DrawImage(ControlImage, 4, 4, Width - 8, Height - 8);
            }
            // Draw text, centered vertically and horizontally
            var rectangle = new RectangleF(2, 0, Width, Height + 5);
            var format = new StringFormat
            {
                LineAlignment = StringAlignment.Center,
                Alignment = StringAlignment.Center
            };
            e.Graphics.DrawString(Text, Font, Brushes.Black, rectangle, format);
        }
    }
}

private void myControl1_Click(object sender, EventArgs e)
       {
           SendKeys.Send("{1}");
       }

private void myControlEnter_Click(object sender, EventArgs e)
       {
           SendKeys.Send("{ENTER}");
       }

[DataMember]
public class MyClass : List<SomeOtherClass>
{
     [DataMember]
     public int SomeIntProp { get; set; }
}

// step 1 - win32 api functions used
[StructLayout(LayoutKind.Sequential)]
public struct COPYDATASTRUCT {
  public IntPtr dwData;
  public int cbData;
  [MarshalAs(UnmanagedType.LPStr)] public string lpData;
}
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]
public static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, ref COPYDATASTRUCT lParam);

public const int WM_COPYDATA = 0x004A;

...

// step 2 - in your Main()
var running_already = Process.GetProcessesByName("Yourapp").ToDictionary(x => x.Id, x => x);
running_already.Remove(Process.GetCurrentProcess().Id);
if (running_already.Count > 0) {
    // there's another instance running
    if (args.Length == 0)
        // just let the other instance run
        return;
    // for the other instance - what to open
    string open = args[0];

    var cds = new win32.COPYDATASTRUCT {
        dwData = new IntPtr(0),
        cbData = open.Length * 2 + 1,
        lpData = open
    };
    var handle = running_already.First().Value.MainWindowHandle;
    win32.SendMessage(handle, win32.WM_COPYDATA, IntPtr.Zero, ref cds);
    return;
}

// step 3 - in your main window
protected override void WndProc(ref Message m) {
	if (m.Msg == win32.WM_COPYDATA) {
		var st = (win32.COPYDATASTRUCT) Marshal.PtrToStructure(m.LParam, typeof (win32.COPYDATASTRUCT));
		string open = st.lpData;
		util.postpone(() => on_file_drop(open), 100);
	}

	base.WndProc(ref m);
}